1. Filed of the Invention
This invention relates to aircraft and ship boundary layer propulsion (thrust generation) mechanisms employing flapping foils. It further relates to aerodynamic or hydrodynamic lift systems employing flapping foils. This invention also relates to aerodynamic and hydrodynamic drag generated on aircraft and ships. More precisely, it relates to aerodynamic/hydrodynamic control systems employing flapping foils, including the delay or prevention of flow separation in general fluid flow systems.
2. Background of the Invention
It is firmly established that fluid flow over most air and marine vehicles occurs at such large Reynolds numbers that the viscous forces acting on the fluid particles are of importance only in a very thin layer surrounding the vehicle. The remainder of the fluid flow field can be regarded as inviscid (frictionless). The Reynolds number signifies the ratio of the fluid particle's inertia force to the viscous force. Typically, the inertia force is several hundred thousand or million times larger than the viscous force. As a result, the thin fluid particle layer (which is usually referred to as "boundary layer") developing on an aircraft wing typically has a thickness of only one or two percent of the aircraft wing's chord length.
Over the years, it has also been recognized that the type of fluid flow which develops in the boundary layer has a decisive effect on the air or marine vehicle's drag. One distinguishes between the two types of fluid flow, namely "laminar flow" and "turbulent flow". The laminar fluid flow is a well-ordered steady flow, whereas the turbulent fluid flow is characterized by small irregular fluctuations about a steady mean flow. The friction force exerted by a laminar flow's boundary layer on the vehicle's outer surface exposed to the laminar flow is significantly smaller than the friction force generated by a turbulent flow's boundary layer. On the other hand, a laminar flow boundary layer has a much greater proclivity to separate (detach) from the vehicle's surface than a turbulent flow boundary layer. Separated flow produces a different type of drag, which is usually referred to as "pressure drag". Large regions of flow separation may produce huge increases in pressure drag. Hence efficient air or marine vehicle design requires the minimization of both skin friction and pressure drag.